nir: prevent use-after-free condition in should_lower_phi()

nir: prevent use-after-free condition in should_lower_phi()

lower_phis_to_scalar() pass recurses the instruction dependence graph to
determine if all the sources of a given instruction are scalarizable.
To prevent cycles, it temporary marks the phi instruction before recursing in,
then updates the entry with the resulting value. However, it does not consider
that the entry value may have changed after a recursion pass, hence causing
a use-after-free situation and a crash.

This patch fixes this by reloading the entry corresponding to the 'phi'
after recursing and before updating its value.

The crash can be reproduced ~20% of times with the dEQP test:

dEQP-GLES3.functional.shaders.loops.while_constant_iterations.nested_sequence_fragment

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>